In rotary printing machines, and especially in sheet-fed offset printing machines having a plurality of printing units, replacing printing plates can be considerably time-consuming. In order to reduce the machine shutdown time needed for setting the printing machine for a new printing job, automatic printing plate exchange devices have been developed in the past. In those automatic printing plate exchange devices, the procedure for feeding a new printing plate is performed automatically via drive means. Generally the feeding procedure begins by removing the new printing plate from a standby position and introducing it into the opening of a clamping rail on the plate cylinder. The leading edge of the printing plate is then clamped in the clamping rail, and the plate cylinder is rotated in the forward direction for approximately one revolution until the trailing end of the printing plate can be gripped by a clamping and tensioning device on the plate cylinder.
When a new printing plate is inserted into the opening of the clamping rail, it is particularly important for the leading edge of the printing plate to assume an in-register position before the clamping rail is closed to secure the leading end of the plate on the plate cylinder. A sheet-fed offset machine typically uses a plurality of printing plates to produce one print. Even if only one of the printing plates is not attached to its associated plate cylinder properly, complex register correction operations have to be carried out to put the printing plate in registry. Thus, if an automatic plate feeding method does not guarantee in-register mounting of printing plates on plate cylinders, the time saved by using the automatic feeding method can be outweighed by the time and effort spent in performing the register correction operations.
German Patent DE 3 940 796 C2 discloses a method and device for the automatic exchange of printing plates. In the method disclosed in this reference, position detection means is used to establish the in-register condition of the leading edge of the printing plate in the opening of the clamping rail. The leading end of the printing plate is clamped in the clamping rail after the leading edge of the printing plate is detected to have assumed an in-register position. As disclosed in this reference, the printing plate being fed to the clamping rail is held between a transporting roller and a back-pressure roller. The transporting roller is driven to advance the printing plate to the clamping rail. If the position detection means indicates that the leading edge of the printing plate is not in an in-register position, the transportation roller will be actuated and the printing plate will be advanced. The steps of position detection and plate advancement are repeated several times until registration occurs or, if the in-register position of the leading edge of the printing plate still does not occur after a number of attempts, the feeding operation is aborted. The disadvantage of this method is that the printing plate may be introduced into the clamping rail with too great a force thereby causing damage to the printing plate or to the other components of the press.
European Patent 0 581 212 A1 discloses an electrical register control which utilizes register pins arranged in the clamping rail for clamping the leading end of the printing plate on the plate cylinder. The register pins are spaced apart in the axial direction of the plate cylinder and are arranged to be electrically insulated from the plate cylinder and the clamping rail. The leading edge of the printing plate has U-shaped notches which are designed to abut the register pins when the plate is in the in-register position. The in-register condition of the plate can be established by detecting the formation of an electrically conducting contact between the electrically conductive leading edge of the printing plate and the register pins. In this reference, the register pins are electrically connected in series. As a result, an electrical connection will only occur when both register pins contact the leading edge of the printing plate, and, therefore, the position of the plate is only detected when both of the U-shaped notches on the leading edge of the plate abut the register pins at the same time. The disadvantage of this arrangement is that unless the printing plate contacts both register pins at the same time (i.e., it is in registry), there is no way to tell whether the leading edge of the plate is not making contact with either register pin, or whether the plate has been introduced obliquely so that it is making contact with only one of the register pins. In the latter case there is also no way to tell which pin is in contact with the leading edge of the plate. German Patent DE 3 527 103 A1 discloses a device for setting the position of a printing plate on a plate cylinder after the plate has been mounted on the plate cylinder. That device has three block-shaped mechanical impact probes disposed on circumference of the plate cylinder. Each impact probe cooperates with a rectangular register hole in the front-end region of the printing plate in such a manner that when a predefined side of the register hole contacts the impact probe, an associated indicator light or an LED is switched on to indicate the formation of the contact. This arrangement can be used to establish the in-register position of a printing plate which is already mounted on the plate cylinder. It is, however, not related to the automatic feeding of printing plates.
German Patent DE 4 226 780 A1 discloses the use of register pins in a device for controlling the in-register positioning of a printing plate on a plate cylinder. The register pins are electrically insulated from the plate cylinder. With such an arrangement, it is possible to establish the in-register condition of the front edge of a printing plate at two axially-separated sites in the clamping rail. In the device disclosed in this reference, each register pin generates an in-register signal when it is in contact with the leading edge of the printing plate. The in-register signal is received by means of a sensing probe which is arranged to contact a contact surface on the register pin.
European Patent 0 551 976 A1 discloses the use of register pins which are electrically insulated from the plate cylinder on which the register pins are disposed. In the European Patent 0 581 212 A1 described above, such register pins are arranged to cooperate with U-shaped notches on the leading edge of the printing plate to generate in-register signals. Because the register pins are electrically connected in series, the position of the printing plate is detected only when the leading edge of the printing plate is in contact with both register pins.